API Documentation

API Documentation

Why Engineers Care About This

API documentation is how developers learn to use your API. Good documentation reduces support burden, speeds up integration, and prevents misuse. Bad documentation frustrates developers, increases support requests, and leads to incorrect API usage. Documentation is often treated as an afterthought, but it's a critical part of API design.

When developers can't figure out how to use your API, or they use it incorrectly, or support is overwhelmed with questions, you're hitting documentation problems. These problems compound. Without good documentation, every new developer integration requires support help. Incorrect API usage causes bugs and data issues. Good documentation prevents these problems by making APIs self-service.

In interviews, when someone asks "How would you document this API?", they're really asking: "Do you understand that documentation is part of API design? Do you know how to create documentation that helps developers? Do you understand that documentation must stay in sync with code?" Most engineers don't. They write documentation once and forget it, or don't write documentation at all.

Core Intuitions You Must Build

• Documentation is part of API design, not an afterthought. Good APIs are self-documenting (clear naming, consistent patterns), but documentation is still necessary. Documentation explains why APIs work the way they do, provides examples, and covers edge cases. Don't treat documentation as optional—it's part of the API contract. Also, documentation should be written alongside code, not after.

• OpenAPI/Swagger enables structured, interactive documentation. OpenAPI (formerly Swagger) is a standard format for API documentation. It defines endpoints, parameters, responses, and examples in a machine-readable format. Tools generate interactive documentation (Swagger UI) from OpenAPI specs, enabling developers to explore and test APIs. Use OpenAPI for structured documentation—it's standard, tool-supported, and enables automation.

• Documentation must stay in sync with code. Outdated documentation is worse than no documentation—it misleads developers. Keep documentation in sync with code. Use code-first approaches (generate OpenAPI from code annotations) or docs-first approaches (generate code from OpenAPI). Either way, documentation and code must match. Also, include documentation updates in code review—don't merge API changes without updating docs.

• Examples are more valuable than descriptions. Developers learn from examples, not descriptions. Include request/response examples for every endpoint. Show common use cases, error cases, and edge cases. Also, include code examples in multiple languages (cURL, JavaScript, Python). Examples make documentation actionable—developers can copy/paste and modify.

• Error documentation is as important as success documentation. APIs return errors, but documentation often only covers success cases. Document error responses—what errors can occur, when they occur, and how to handle them. Include error response examples and error codes. This helps developers handle errors correctly and reduces support requests.

• Interactive documentation enables testing and exploration. Static documentation (Markdown, PDF) is hard to explore and test. Interactive documentation (Swagger UI, Postman) lets developers explore endpoints, test requests, and see responses. Use interactive documentation when possible—it reduces friction and enables self-service integration.

Subtopics (Taught Through Real Scenarios)

OpenAPI/Swagger Specification

What people usually get wrong:

Engineers often think "documentation is just writing descriptions." But structured documentation (OpenAPI/Swagger) enables tooling—interactive docs, code generation, testing, validation. OpenAPI defines APIs in a machine-readable format, enabling automation. Use OpenAPI for structured documentation—it's standard, tool-supported, and enables better developer experience.

How this breaks systems in the real world:

A service had API documentation in a wiki (Markdown). The documentation was hard to navigate, couldn't be tested, and was often outdated. Developers had to read through pages of text to find endpoints, then manually test them. Integration was slow and error-prone. The fix? Use OpenAPI/Swagger—generate interactive documentation (Swagger UI) from OpenAPI spec. Now developers can explore endpoints, test requests, and see responses. But the real lesson is: structured documentation enables tooling. OpenAPI enables interactive docs, code generation, and testing.

What interviewers are really listening for:

They want to hear you talk about OpenAPI/Swagger, structured documentation, and tooling. Junior engineers say "just write documentation in Markdown." Senior engineers say "use OpenAPI/Swagger for structured documentation—it enables interactive docs, code generation, and testing, making APIs easier to use." They're testing whether you understand that documentation is about developer experience, not just "writing descriptions."

Keeping Documentation in Sync

What people usually get wrong:

Engineers often write documentation once and forget it. But APIs change, and documentation must change with them. Outdated documentation misleads developers and causes incorrect API usage. Keep documentation in sync with code. Use code-first approaches (generate OpenAPI from code annotations) or docs-first approaches (generate code from OpenAPI). Either way, documentation and code must match.

How this breaks systems in the real world:

A service updated an API endpoint (changed parameter name) but didn't update documentation. Developers used the old parameter name (from documentation), causing errors. Support was overwhelmed with questions. The fix? Include documentation updates in code review—don't merge API changes without updating docs. Also, use code-first approaches (generate OpenAPI from code) to keep docs in sync automatically. But the real lesson is: documentation must stay in sync with code. Outdated documentation is worse than no documentation.

What interviewers are really listening for:

They want to hear you talk about keeping documentation in sync, code-first vs docs-first approaches, and documentation in code review. Junior engineers say "just update documentation when you remember." Senior engineers say "keep documentation in sync with code—use code-first (generate OpenAPI from code) or docs-first (generate code from OpenAPI) approaches, and include documentation updates in code review." They're testing whether you understand that documentation maintenance is part of API development.

Examples and Use Cases

What people usually get wrong:

Engineers often document endpoints with descriptions but no examples. But developers learn from examples, not descriptions. Include request/response examples for every endpoint. Show common use cases, error cases, and edge cases. Also, include code examples in multiple languages (cURL, JavaScript, Python). Examples make documentation actionable.

How this breaks systems in the real world:

An API had documentation with descriptions but no examples. Developers had to guess request formats, causing errors. Integration was slow and error-prone. The fix? Add request/response examples for every endpoint, including error cases. Also, add code examples in multiple languages. Now developers can copy/paste and modify examples. But the real lesson is: examples are more valuable than descriptions. Include examples for every endpoint.

What interviewers are really listening for:

They want to hear you talk about examples, use cases, and code samples. Junior engineers say "just describe the endpoint." Senior engineers say "include request/response examples for every endpoint, show common use cases and error cases, and provide code examples in multiple languages—examples make documentation actionable." They're testing whether you understand that documentation is about helping developers, not just describing APIs.

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• Documentation is part of API design—write it alongside code, not after
• OpenAPI/Swagger enables structured, interactive documentation—use it for better developer experience
• Documentation must stay in sync with code—use code-first or docs-first approaches
• Examples are more valuable than descriptions—include request/response examples for every endpoint
• Error documentation is as important as success documentation—document error responses and handling
• Interactive documentation enables testing and exploration—use Swagger UI or similar tools
• Good documentation reduces support burden—make APIs self-service with clear documentation

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• API Design - Designing APIs that are easy to document
• Error Handling & Logging - Documenting error responses